Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 11 Feb 2020 22:27:54 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc:     Petr Mladek <pmladek@suse.com>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dmitry Monakhov <dmtrmonakhov@yandex-team.ru>,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Subject: Re: [PATCH] kernel/watchdog: flush all printk nmi buffers when
 hardlockup detected
Message-ID: <20200211222754.0185b9b3@rorschach.local.home>
In-Reply-To: <20200212030403.GC13208@google.com>
References: <158132813726.1980.17382047082627699898.stgit@buzz>
        <20200212011551.GA13208@google.com>
        <20200211214958.5d8f4004@rorschach.local.home>
        <20200212030403.GC13208@google.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, 12 Feb 2020 12:04:03 +0900
Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> wrote:

> Oh, yes, ftrace printks a lot. But I sort of forgot why don't we do
> the same for "regular" NMIs. So NMIs use per-cpu buffers, expect for
> NMIs which involve ftrace dump. I'm missing something here.

Well, ftrace_dump() is called from NMI context when the system is about
to crash (ftrace_dump_on_oops). And at that moment, we care more about
the trace than other output (it's most like to contain the information
that caused the bug).

But for things like sysrq-l, that does a printk in NMI context for
normal operations, we don't want strange races to occur and affect
other printk operations. Having them in buffers controls the output a
bit better.

But with the new printk ring buffer work, perhaps that's no longer
necessary.

-- Steve